The power characteristic of a four-cycle engine is determined to a significant extent by the quality of the charge cycle. In a combustion engine, the exhaust valve practically always opens the cylinder at a supercritical pressure gradient, so the flow processes in the charge cycle channels are highly non-steady. When the exhaust valve is opened before reaching bottom dead center (BDC) of the piston, a pre-exhaust wave, i.e., an exhaust process with a supercritical pressure gradient, is generated. Since the exhaust port monitored by the exhaust valve of a combustion engine can be regarded as a poorly designed Laval nozzle, a flow discontinuity occurs due to the non-steady-state flow conditions. This always results in the development of a perpendicular compression wave. Due to the perpendicular compression wave, flow is immediately decelerated to subsonic speed so the exhaust mass flow drops greatly. In addition, due to the pre-exhaust wave, aerodynamic noises are generated, resulting in corresponding pressure fluctuations. To counteract the inadequacies described above, in particular in the pre-exhaust wave, numerous studies have been conducted with regard to improving the charge cycle load processes.
For example, a computational method and a program for the aerodynamic supersonic flow of the exhaust and for the shaping of the valve were developed as described in MTZ (Motor Technische Zeitschrift [Automotive Engineering Journal] 51, 1990, No. 7/8, pp. 336 to 343). The goal was to design the exhaust port and/or the valve geometry so that the calculated system will function without waves or pulsation. The possibility of completing the flow by reducing the exhaust velocities of the combustion gases was also taken into account.